Multiplicate sensitization of novel near-infrared luminescent linear copolymers based on Er, Nd and Yb-complexes

A series of novel near-infrared (NIR) luminescent linear copolymers, PCzLnQL(2) (Cz_carbazole; Ln_Er, Nd, and Yb; Q = 8-hydroxyquinoline; L = 8-hydroxyquinoline, 5-[(carbazole-9-yl) methyl]-8-hydroxyquinoline and 2-thenoyltrifluoroacetone) covalently linked with Ln-complexes were synthesized and characterized. The obtained copolymers exhibit appropriate molecular weight, good solubility-processability and thermal stability for PLEDs application. The photophysical properties of the copolymers were studied by UV-vis absorption, steady state and transient fluorescence spectra. Monitoring the characteristic emission of the corresponding Ln(3+), the copolymers show a broad excitation band extending from UV light to the region of visible light. Upon excitation with UV-vis light, the copolymers display high efficient near-infrared (NIR) luminescence of the corresponding Ln(3+). Effects of N-vinylcarbazole segments and ligands on the NIR-luminescence of Ln(3+) were investigated carefully. The results indicate that the NVK segments are not only used as light-harvesting groups, but also as a barrier to form a special microenvironment for the Ln-complex moieties. The three ligands can sensitize the three lanthanide ions, but the sensitized efficiency is not identical. Based on the observed luminescence phenomena and the reported energy transfer theories, the energy transfer mechanism for the NIR-luminescence of the copolymers was also investigated, which will provide a rule for designing perfect NIR-luminescence materials.